Photomultiplier

Abstract: A photomultiplier comprising an electron multiplier for minimizing a variation in multiplication factor and noise is characterized in that insulating members (80a,b) are aligned on the same line (606) to insulate a plurality of dynode plates for constituting a dynode unit from each other, thereby preventing a damage to each dynode plate. At the same time, a through hole (600) is formed to fix the insulating member provided to each dynode plate such that a gap is provided between the major surface of the dynode plate and the surface of the insulating member, thereby preventing discharge between dynode plates, which is caused due to dust or the like deposited on the surface of the insulating member.

Description: The invention relates to an electron multiplier and a photomultiplier. Related Background Art In a conventional electron multiplier, a plurality of dynodes are multilayered at predetermined intervals to constitute a dynode unit for cascade-multiplying an incident electron flow. In U.S. Patent No. US-A-3,229,143, insulating balls are inserted between dynodes to constitute a dynode unit. Fig. 1 shows the main part of this structure. A through hole 103 is formed in each support plate 101 for supporting the corresponding stage of dynodes. An insulating ball 102 having part thereof fit in the opening ends of the through holes 103 is inserted between the support plates 101. The insulating ball 102 is formed of pyrex and has a diameter larger than the inner diameter of the through hole 103. On the other hand, the through hole 103 forms a cylindrical hole having a predetermined inner diameter. In the conventional structure, an acute- or right-angled edge portion (contact portion to the insulating member 102) is formed at the opening end of the through hole 103. When this portion is brought into contact with the insulating ball 102, pressed in a stacking direction, and deformed, burrs can be formed at the edge portion. When the edge portion which is in contact with the insulating ball 102 is deformed, the distance between the adjacent support plates 101 decreases. Even if this phenomenon slightly occurs at the edge portions of all the through holes 103, the intervals between the dynodes vary to cause a variation in multiplication factor (gain). In addition, due to those burrs, a field concentration occurs at the edge portions to generate noise. When a force is applied to the insulating balls 102 in the stacking direction, a pressure is applied to the support plates 101 through the insulating balls 102. As a result, the dynodes formed integral with the support plates 101 are deflected. This also makes the intervals between the dynodes nonuniform. The invention aims to provide an electron multiplier and photomultiplier structure capable of solving the aforementioned problems. The invention aims to provide a structure in which plates for supporting dynodes are held at predetermined intervals to minimize a variation in multiplication factor and noise and prevent discharge between the dynode plates. In one aspect the invention provides an electron multiplier comprising: an anode plate; and a dynode unit comprising a plurality of dynode plates so stacked with said anode plate that the last dynode plate of said dynode unit opposes said anode plate, said dynode plates being spaced apart from each other at predetermined intervals and supported in the stack by way of insulating members to enable the dynode unit to effect cascade-multiplying of electrons incident thereon; characterized in that: said insulating members are arranged in a sequence with each insulating member being in direct contact with its adjacent insulating member or members via respective through-holes in said dynode plates, each adjacent pair of insulating members defines therebetween an interstice, and each dynode plate is supported within a respective interstice. In another aspect the invention provides a photomultiplier as aforementioned, the photomultiplier further comprising a photocathode for receiving photons and emitting photoelectrons, said dynode unit being positioned between said photocathode and said anode plate for receiving photoelectrons emitted by said photocathode. In an embodiment of the invention to be described hereinafter, the electron multiplier is mounted on a base member and arranged in a housing formed integral with the base member for fabricating a vacuum container. The photocathode is arranged inside the housing and deposited on the surface of a light receiving plate provided to the housing. At least one anode is supported by an anode plate and arranged between the dynode unit and the base member. The dynode unit is constituted by stacking a plurality of stages of dynode plates for respectively supporting at least one dynode for receiving and cascade-multiplying photoelectrons emitted from the photocathode in an incidence direction of the photoelectrons. The housing may have an inner wall thereof deposited a conductive metal for applying a predetermined voltage to the photocathode and rendered conductive by a predetermined conductive metal to equalize the potentials of the housing and the photocathode. The photomultiplier embodying the invention has at least one focusing electrode between the dynode unit and the photocathode. The focusing electrode is supported by a focusing electrode plate. The focusing electrode plate is fixed on the electron incident side of the dynode unit through insulating members. The focusing electrode plate has holding springs and at least one contact terminal, all of which are integrally formed with this plate. The holding springs are in contact with the inner wall of the housing to hold the arrangement position of the dynode unit fixed on the focusing electrode plate through the insulating members. The contact terminal is in contact with the photocathode to equalize the potentials of the focusing electrodes and the photocathode. The contact terminal functions as a spring. A plurality of anodes may be provided to the anode plate, and electron passage holes through which secondary electrons pass are formed in the anode plate in correspondence with positions where the secondary electrons emitted from the last-stage of the dynode unit reach. Therefore, the photomultiplier has, between the anode plate and the base member, an inverting dynode plate for supporting at least one inverting dynode in parallel to the anode plate. The inverting dynode plate inverts the orbits of the secondary electrons passing through the anode plate toward the anodes. The diameter of the electron incident port (dynode unit side) of the electron passage hole formed in the anode plate is smaller than that of the electron exit port (inverting dynode plate side). The inverting dynode plate has, at positions opposing the anodes, a plurality of through holes for injecting a metal vapor to form at least a secondary electron emitting layer on the surface of each dynode of the dynode unit. On the other hand, the photomultiplier embodying the invention may have, between the inverting dynode plate and the base member, a shield electrode plate for supporting at least one shield electrode in parallel to the inverting dynode plate. The shield electrode plate inverts the orbits of the secondary electrons passing through the anode plate toward the anodes. The shield electrode plate has a plurality of through holes for injecting a metal vapor to form at least a secondary electron emitting layer on the surface of each dynode of the dynode unit. In place of this shield electrode plate, a surface portion of the base member opposing the anode plate may be used as an electrode and substituted for the shield electrode plate. In particular, the electron multiplier comprises a dynode unit constituted by stacking a plurality of stages of dynode plates, the dynode plates spaced apart from each other at predetermined intervals through insulating members in an incidence direction of the electron flow, for respectively supporting at least one dynode for cascade-multiplying an incident electron flow, and an anode plate opposing the last-stage dynode plate of the dynode unit through insulating members. Each dynode plate has a first concave portion for arranging a first insulating member which is provided on the first main surface of the dynode plate and partially in contact with the first concave portion and a second concave portion for arranging a second insulating member which is provided on the second main surface of the dynode plate and partially in contact with the second concave portion (the second concave portion communicates with the first concave portion through a through hole). The first insulating member arranged on the first concave portion and the second insulating member arranged on the second concave portion are in contact with each other in the through hole. An interval between the contact portion between the first concave portion and the first insulating member and the contact portion between the second concave portion and the second insulating member is smaller than that between the first and second main surfaces of the dynode plate. The above concave portion can be provided in the anode plate, the focusing plate, inverting dynode plate and the shield electrode plate. The following points should be noted. The first point is that gaps are formed between the surface of the first insulating member and the main surface of the first concave portion and between the second insulating member and the main surface of the second concave portion, respectively, to prevent discharge between the dynode plates. The second point is that the central point of the first insulating member, the central point of the second insulating member, and the contact point between the first and second insulating members are aligned on the same line in the stacking direction of the dynode plates so that the intervals between the dynode plates can be sufficiently kept. Using spherical or circularly cylindrical bodies as the first and second insulating members, the photomultiplier can be easily manufactured. When circularly cylindrical bodies are used, the outer surfaces of these bodies are brought into contact with each other. The shape of an insulating member is not limited to this. For example, an insulating member having an elliptical or polygonal section can also be used as long as the object of the present invention can be achieved. In this electron multiplier, each dynode plate has an engaging member at a predetermined position of a side surface of the plate to engage with a corresponding connecting pin for applying a predetermined voltage. Therefore, the engaging member is projecting in a vertical direction to the incident direction of the photoelectrons. The engaging member is constituted by a pair of guide pieces for guiding the connecting pin. On the other hand, a portion near the end portion of the connecting pin, which is brought into contact with the engaging member, may be formed of a metal material having a rigidity lower than that of the remaining portion. Each dynode plate is constituted by at least two plates, each having at least one opening for forming as the dynode and integrally formed by welding such that the openings are matched with each other to function as the dynode when the two plates are overlapped. To integrally form these two plates by welding, each of the plates has at least one projecting piece for welding the corresponding two plates. The side surface of the plate is located in parallel with respect to the incident direction of the photoelectrons. The insulating member having a spherical shape or the like is in contact with the concave portion formed in each dynode plate. The insulating members are in contact with each other in the through hole extending through the seat holes formed in the main surfaces of the dynode plates. With this structure, the following effects can be obtained. A force applied in the stacking direction is mostly received by the series of insulating members, and no excess force is applied to the dynode plates. Since the insulating member is in contact with the seat holes in the dynode plates, the centers of the upper and lower insulating members coincide with the central portion of the through hole. As a result, positioning of the dynode plates in the horizontal direction can be easily performed. In addition, the edge portion of the opening is not pressed and deformed as in the prior art. The contact portion between the insulating member and the concave portion is positioned in the direction of thickness of the dynode plate rather than the main surface of the dynode plate having the concave portion. Therefore, the intervals between the dynode plates can be substantially increased (Figs. 8 and 9). Discharge between the dynode plates is often caused due to dust or the like deposited on the surface of the insulating member. However, in the structure according to the present invention, intervals between the dynode plates are substantially increased, thereby obtaining a structure effective to prevent the discharge. The present invention will become more fully understood from consideration of a detailed description of embodiments given hereinbelow with reference to the accompanying drawings. The description and drawings are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the claims will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing the structure of a conventional electron multiplier; Fig. 2 is a partially cutaway perspective view showing the entire structure of a photomultiplier embodying the present invention; Fig. 3 is a sectional view showing a typical shape of a concave portion formed in a dynode plate in the photomultiplier of Fig. 2; Fig. 4 is a sectional view showing the first shape of the concave portion as a first application of the concave portion shown in Fig. 3; Fig. 5 is a sectional view showing the second shape of the concave portion as a second application of the concave portion shown in Fig. 3; Fig. 6 is a sectional view showing the third shape of the concave portion as a third application of the concave portion shown in Fig. 3; Fig. 7 is a sectional view showing the fourth shape of the concave portion as a fourth application of the concave portion shown in Fig. 3; Fig. 8 is a sectional view showing the structure between dynode supporting members in the conventional photomultiplier as a comparative example; Fig. 9 is a sectional view showing the structure between the dynode plates; Fig. 10 is a sectional side view showing the simple internal structure of the photomultiplier, in which a metal housing is cut; Fig. 11 is a plan view showing the photomultiplier of Figs. 2 and 10; Fig. 12 is a sectional side view particularly showing an electron multiplier in the photomultiplier shown in Fig. 10; Fig. 13 is an enlarged sectional view showing part of a dynode unit; Fig. 14 is an enlarged perspective view showing the first structure of the dynode plate and an insulating member; and Fig. 15 is an enlarged perspective view showing the second structure of the dynode plate and an insulating member. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Fig. 2 is a perspective view showing the entire structure of a photomultiplier embodying the present invention. Referring to Fig. 2, the photomultiplier is basically constituted by a photocathode 3 and an electron multiplier. The electron multiplier includes anodes (anode plate 5) and a dynode unit 60 arranged between the photocathode 3 and the anodes. The electron multiplier is mounted on a base member 4 and arranged in a housing 1 which is formed integral with the base member 4 to fabricate a vacuum container. The photocathode 3 is arranged inside the housing 1 and deposited on the surface of a light receiving plate 2 provided to the housing 1. The anodes are supported by the anode plate 5 and arranged between the dynode unit 60 and the base member 4. The dynode unit 60 is constituted by stacking a plurality of stages of dynode plates 6, for respectively supporting a plurality of dynodes 603 (Fig. 3) for receiving and cascade-multiplying photoelectrons emitted from the photocathode 3, in the incidence direction of the photoelectrons. The photomultiplier also has focusing electrodes 8 between the dynode unit 60 and the photocathode 3 for correcting orbits of the photoelectrons emitted from the photocathode 3. These focusing electrodes 8 are supported by a focusing electrode plate 7. The focusing electrode plate 7 is fixed on the electron incidence side of the dynode unit 60 through insulating members 8a and 8b. The focusing electrode plate 7 has holding springs 7a and contact terminals 7b, all of which are integrally formed with this plate 7. The holding springs 7a are in contact with the inner wall of the housing 1 to hold the arrangement position of the dynode unit 60 fixed on the focusing electrode plate 7 through the insulating members 8a and 8b. The contact terminals 7b are in contact with the photocathode 3 to equalize the potentials of the focusing electrodes 8 and the photocathode 3 and functions as springs. When the focusing electrode plate 7 has no contact terminal 7b, the housing 1 may have an inner wall thereof deposited a conductive metal for applying a predetermined voltage to the photocathode 3, and the contact portion between the housing 1 and the photocathode 3 may be rendered conductive by a predetermined conductive metal 12 to equalize the potentials of the housing 1 and the photocathode 3. Although both the contact terminals 7b and the conductive metal 12 are illustrated in Fig. 2, one structure can be selected and realized in an actual implementation. The anode is supported by the anode plate 5. A plurality of anodes may be provided to this anode plate 5, and electron passage holes through which secondary electrons pass are formed in the anode plate 5 in correspondence with positions where the secondary electrons emitted from the last-stage dynode of the dynode unit 60 reach. Therefore, this photomultiplier has, between the anode plate 5 and the base member 4, an inverting dynode plate 13 for supporting inverting dynodes in parallel to the anode plate 5. The inverting dynode plate 13 inverts the orbits of the secondary electrons passing through the anode plate 5 toward the anodes. The diameter of the electron incident port (dynode unit 60 side) of the electron passage hole formed in the anode plate 5 is smaller than that of the electron exit port (inverting dynode plate 13 side). The inverting dynode plate 13 has, at positions opposing the anodes, a plurality of through holes for injecting a metal vapor to form a secondary electron emitting layer on the surface of each dynode 603 of the dynode unit 60. On the other hand, the photomultiplier may have, between the inverting dynode plate 13 and the base member 4, a shield electrode plate 14 for supporting sealed electrodes in parallel to the inverting dynode plate 13. The shield electrode plate 14 inverts the orbits of the secondary electrons passing through the anode plate 5 toward the anodes. The shield electrode plate 14 has a plurality of through holes for injecting a metal vapor to form a secondary electron emitting layer on the surface of each dynode 603 of the dynode unit 60. In place of this shield electrode plate 14, a surface portion 4a of the base member 4 opposing the anode plate 5 may be used as a sealed electrode and substituted for the shield electrode plate 14. In particular, the electron multiplier comprises a dynode unit 60 constituted by stacking a plurality of stages of dynode plates 6, spaced apart from each other at predetermined intervals by the insulating members 8a and 8b in the incidence direction of the electron flow, and each dynode plate 6 is supporting a plurality of dynodes 603 for cascade-multiplying an incident electron flow, and the anode plate 5 opposing the last-stage dynode plate 6 of the dynode unit 60 through the insulating members 8a and 8b. In this electron multiplier, each dynode plate 6 has an engaging member 9 at a predetermined position of a side surface of the plate to engage with a corresponding connecting pin 11 for applying a predetermined voltage. The side surface of the dynode plate 6 is in parallel with respect to the incident direction of the photoelectrons. The engaging member 9 is constituted by a pair of guide pieces 9a and 9b for guiding the connecting pin 11. The engaging member may have a hook-like structure (engaging member 99 illustrated in Fig. 2). The shape of this engaging member is not particularly limited as long as the connecting pin 11 is received and engaged with the engaging member. On the other hand, a portion near the end portion of the connecting pin 11, which is brought into contact with the engaging member 9, may be formed of a metal material having a rigidity lower than that of the remaining portion. Each dynode plate 6 used is constituted by two plates 6a and 6b having openings for forming the dynodes and integrally formed by welding such that the openings are matched with each other to function as dynodes when the two plate are overlapped each other. To integrally form the two plates 6a and 6b by welding, the two plates 6a and 6b have projecting pieces 10 for welding the corresponding projecting pieces thereof at predetermined positions matching when the two plates 6a and 6b are overlapped each other. The structure of each dynode plate 6 for constituting the dynode unit 60 will be described below. Fig. 3 is a sectional view showing the shape of the dynode plate 6. Referring to Fig. 3, the dynode plate 6 has a first seat hole 601a for arranging a first insulating member 80a which is provided on a first main surface of the dynode plate 6 and partially in contact with the first concave portion 601a and a second concave portion 601b for arranging a second insulating member 80b which is provided on a second main surface of the dynode plate 6 and partially in contact with the second concave portion 601b (the second concave portion 601b communicates with the first concave portion 601 through a through hole 600). The first insulating member 80a arranged on the first concave portion 601a and the second insulating member 80b arranged on the second concave portion 601b are in contact with each other in the through hole 600. An interval between the contact portion 605a between the first concave portion 601a and the first insulating member 80a and the contact portion 605b of the second concave portion 601b and the second insulating member 80b is smaller than that (thickness of the dynode plate 6) between the first and second main surfaces of the dynode plate 6. Gaps 602a and 602b are formed between the surface of the first insulating member 80a and the main surface of the first concave portion 601a and between the second insulating member 80b and the main surface of the second concave portion 601b, respectively, to prevent discharge between the dynode plates 6. A central point 607a of the first insulating member 80a, a central point 607b of the second insulating member 80b, and a contact point 606 between the first and second insulating members 80a and 80b are aligned on the same line 604 in the stacking direction of the dynode plates 6 so that the intervals between the dynode plates 6 can be sufficiently kept. Using the spherical bodies 8a or circularly cylindrical bodies 8b are used as the first and second insulating members 80a and 80b (insulating members 8a and 8b in Fig. 2), the photomultiplier can be easily manufactured. When circularly cylindrical bodies are used, the side surfaces of the circularly cylindrical bodies are brought into contact with each other. The shape of the insulating member is not limited to this. For example, an insulating member having an elliptical or polygonal section can also be used as long as the object of the present invention can be achieved. Referring to Fig. 3, reference numeral 603 denotes a dynode. A secondary electron emitting layer containing an alkali metal is formed on the surface of this dynode. The shapes of the concave portion will be described below with reference to Figs. 4 to 7. For the sake of descriptive convenience, only the first main surface of the dynode plate 6 is disclosed in Figs. 4 to 7. The first concave portion 601a is generally constituted by a surface having a predetermined taper angle (α) with respect to the direction of thickness of the dynode plate 6, as shown in Fig. 4. This first concave portion 601a may be constituted by a plurality of surfaces having predetermined taper angles (α and β) with respect to the direction of thickness of the dynode plate 6, as shown in Fig. 5. The surface of the first concave portion 601a may be a curved surface having a predetermined curvature, as shown in Fig. 6. The curvature of the surface of the first concave portion 601a is set smaller than that of the first insulating member 80a, thereby forming the gap 602a between the surface of the first concave portion 601a and the surface of the first insulating member 80a. To obtain a stable contact state with respect to the first insulating member 80a, a surface to be brought into contact with the first insulating member 80a may be provided to the first concave portion 601a, as shown in Fig. 7. In this embodiment, a structure having a high mechanical strength against a pressure in the direction of thickness of the dynode plate 6 even compared to the above-described structures in Figs. 4 to 6 can be obtained. The detailed structure between the dynode plates 6, adjacent to each other, of the dynode unit 60 will be described below with reference to Figs. 8 and 9. Fig. 8 is a partial sectional view showing the conventional photomultiplier as a comparative example of the present invention. Fig. 9 is a partial sectional view showing the photomultiplier embodying the present invention. In the comparative example shown in Fig. 8, the interval between the support plates 101 having no concave portion is almost the same as a distance A (between contact portions E between the support plates 101 and the insulating member 102) along the surface of the insulating member 102. On the other hand, in an embodiment of the present invention shown in Fig. 9, since concave portions are formed, a distance B (between the contact portions E between the plates 6a and 6b and the insulating member 8a) along the surface of the insulating member 8a is larger than the interval between plates 6a and 6b. Generally, discharge between the plates 6a and 6b is assumed to be caused along the surface of the insulating member 8a due to dust or the like deposited on the surface of the insulating member 8a. Therefore, as shown in this embodiment (Fig. 9), when the concave portions are formed, the distance B along the surface of the insulating member 8a substantially increases as compared to the interval between the plates 6a and 6b, thereby preventing discharge which occurs when the insulating member 8a is inserted between the plates 6a and 6b. Figs. 10 and 11 are sectional and plan views, respectively, showing the photomultiplier according to this embodiment. In this photomultiplier, a vacuum container is fabricated by the circular light receiving plate 2 for receiving the incident light, the cylindrical metal housing 1 disposed along the outer circumference of the light receiving plate 2, and the circular stem 4 for constituting the base member. The electron multiplier for cascade-multiplying the incident electron flow is disposed in this vacuum container. This electron multiplier includes the dynode unit 60 and the anodes supported by the anode plate 5. The photocathode 3 is provided on the lower surface of the light receiving plate 2. The focusing electrode plate 7 for supporting the focusing electrodes 8 is disposed between the photocathode 3 and the electron multiplier. Therefore, the orbits of the photoelectrons emitted from the photocathode 3 are focused and incident on a predetermined region of the electron multiplier by the focusing electrodes 8. In the electron multiplier, the dynode unit 60 is constituted by stacking a plurality of stages of dynode plates 6 for respectively supporting the dynodes, and the anode plate 5 for supporting the anodes and the inverting dynode plate 13 for supporting the inverting dynodes are sequentially disposed under the dynode unit 60. Twelve connecting pins 11 which are connected to external voltage applying terminals to apply a predetermined voltage to the dynode plates 6 and 13 extend through the stem 4 serving as the base member. Each connecting pin 11 is fixed to the stem 4 at a predetermined portion by hermetic glass 15. The length from the stem 4 to the distal end of each connecting pin 11 changes depending on the dynode plates to be connected. The distal end of each connecting pin 11 is resistance-welded to the connecting terminal (engaging member 9) of the corresponding dynode plate 6. Fig. 12 is an enlarged sectional view particularly showing the electron multiplier in this photomultiplier. The focusing electrode plate 7 for supporting the focusing electrodes 8, the dynode plates 6 for supporting the dynodes 603 for constituting the electron multiplier, the inverting dynode plate 13, and the anode plate 5 for supporting the anodes are stacked at predetermined intervals through the ceramic insulating balls 8a. The plurality of insulating balls 8a are arranged along the edges of the dynode plates 6. Fig. 13 is an enlarged sectional view showing the dynode unit 60. Each dynode plate 6 is constituted by an upper electrode (first plate 6a) and a lower electrode (second plate 6b) which are bonded each other. The dynode 603 having a curved inner surface is formed in the plates 6a and 6b. The through hole 600 which extends from the concave portion 601a of the first plate 6a to the concave portion 601b of the second plate 6b is formed at a portion where the insulating ball 8a is disposed. Therefore, the upper and lower portions of the insulating balls 8a are fit in the concave portion 601a of the upper-stage dynode plate 6 and the concave portion 601b of the lower-stage dynode plate 6, respectively (Fig. 14), to engage with the upper- and lower-stage dynode plates 6. In the through hole 600, the upper and lower insulating balls 8a are in contact with each other. As a result, the central points of the series of insulating balls 8a are aligned on the same line 604. In all dynode plates 6, the through hole 600 has a uniform diameter, the concave portions 601a and 601b have the same size, and the surfaces of the concave portions have the same taper angle with respect to the line 604. The insulating balls 8a opposing each other also have the same size (diameter). Therefore, the central axis of the through holes 600 always matches the central points of the insulating balls 8a. As a result, the dynode plates 6 are not displaced from the inverting dynode plate 13 in the horizontal direction, and predetermined intervals can be obtained. In this embodiment, the insulating balls 8a having a diameter of 0.66 mm are used, and the interval between the dynode plates 6 which are adjacent in the vertical direction is 0.25 mm. With this structure, the dynode plates 6, the inverting dynode plate 13, the anode plate 5, and the focusing electrode plate 7 can be easily and correctly assembled. The distance between the dynode plates 6 along the surface of the insulating ball 8a increases as compared to the prior art (Figs. 8 and 9). As a result, discharge which occurs along the surface of the insulating member 8a can be prevented to reduce the noise caused due to this discharge. In this embodiment, the insulating ball 8a is used as an insulating spacer. However, it is not limited to the ball, and a circularly cylindrical insulating body 8b may be formed, as shown in Fig. 15. Also with this shape, the same function and effect can be obtained. In this case, the corresponding concave portions 601a and 601b of the dynode plates 6 can be formed to have shapes/positions which fit to the outer surface of this circularly cylindrical body 8b. In addition, in this embodiment, a concave portion is formed in the dynode plate 6 for supporting the dynodes. However, a similar concave portion may be formed at a predetermined position of a member for constituting a single dynode. In the photomultiplier embodying the present invention, an insulating spacer disposed between the two dynode plates is formed into a spherical or circularly cylindrical body (to be referred to as the spherical body or the like hereinafter), and the spherical body or the like is received by the side surfaces of the concave portions formed in the dynode plates. With this structure, the contact portion with respect to the spherical body or the like is not pressed and deformed, unlike in the prior art. The spherical bodies are brought into contact with each other in the through hole. For this reason, even when a force is applied to the spherical body or the like in the stacking direction, this force is mostly applied to a series of spherical bodies or the like to prevent the deformation of the dynode plates. Therefore, predetermined intervals between the dynode plates can be kept. Since no burr is formed at the edge portion of the through hole, unlike in the prior art, the noise caused due to the field concentration is reduced, and a variation in multiplication factor can also be minimized. The center of each ball or the like matches with the center of each through hole when the dynode plates are stacked. Therefore, deviations of the dynode plates in the horizontal direction can be prevented to minimize the variation in multiplication factor. In the prior art, the edge portion of the through hole is in direct contact with the spherical body. However, in the present invention, the side surfaces of the concave portions formed in the dynode plates are brought into contact with the spherical body or the like. Therefore, the distance between the dynode plates along the surface of the spherical body can be increased as compared to the prior art. For this reason, discharge along the surface of the ball can be prevented to minimize the noise. From the embodiment thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the following claims.

Claims: An electron multiplier comprising: an anode plate (5); and a dynode unit (60) comprising a plurality of dynode plates (6) so stacked with said anode plate that the last dynode plate of said dynode unit (60) opposes said anode plate (5), said dynode plates (6) being spaced apart from each other at predetermined intervals and supported in the stack by way of insulating members (8a, 8b, 80a, 80b) to enable the dynode unit to effect cascade-multiplying of electrons incident thereon; characterized in that: said insulating members (8a, 8b, 80a, 80b) are arranged in a sequence with each insulating member being in direct contact with its adjacent insulating member or members via respective through-holes (600) in said dynode plates (6), each adjacent pair of insulating members defines therebetween an interstice, and each dynode plate is supported within a respective interstice. An electron multiplier according to claim 1, wherein each said dynode plate (6) comprises: a first depression (601a) formed at a first main surface of the dynode plate for receiving a first insulating member (80a) which makes contact with said first depression at a first contact portion (605a); a second depression (601b) formed at a second main surface of the dynode plate for receiving a second insulating member (80b) which makes contact with said second depression at a second contact portion (605b), said second depression (601b) communicating with said first depression (601a) through said through hole (600), and wherein the internal section of the through hole between the first contact portion (605a) and the second contact portion (605b) is smaller than that of the through hole between said first and second main surfaces. An electron multiplier according to claim 2, wherein respective gaps (602a, 602b) are so formed between said first insulating member (80a) and said first depression (601a) and between said second insulating member (80b) and said second depression (601b) as to prevent discharge between adjacent dynode plates (6). An electron multiplier according to any preceding claim, wherein the sequence of insulating members is so arranged that a central point (607a) of one insulating member (80a), a central point (607b) of another insulating member (80b), and a contact point (606) between said first and second insulating members (80a, 80b) are co-linear. An electron multiplier according to any preceding claim, further comprising a focusing electrode plate (7) for supporting at least one focusing electrode (8) at said dynode unit (60), for receiving electrons incident thereat and for correcting orbits of said incident electrons, said focusing electrode plate (7) being fixed at a first-stage dynode plate (6) of said dynode unit (60) by way of an insulating member, said focusing electrode plate (7) having a depression on a main surface thereof opposing said first-stage dynode plate for receiving said insulating member in contact therewith at a contact portion, the first stage dynode plate (6) having a depression on a main surface thereof opposing said focusing electrode plate (7) for receiving said insulating member in contact therewith at a contact portion, and wherein the distance between the contact portion of said focusing electrode plate (7) and the contact portion of said first-stage dynode plate (6) is larger than that between said focusing electrode plate (7) and said first-stage dynode plate (6). An electron multiplier according to any preceding claim, wherein said anode plate (5) has formed therein electron passage holes through which secondary electrons pass, the holes being formed at positions where secondary electrons from a last-stage dynode plate (6) of said dynode unit (60) will be emitted, said anode plate (5) having a depression on a main surface thereof opposing said last-stage dynode plate (6) for receiving an insulating member in contact therewith at a contact portion, the last-stage dynode plate having a depression on a main surface thereof opposing said anode plate for receiving said insulating member in contact therewith at a contact portion, and wherein the distance between the contact portion of said anode plate (5) and the contact portion of said last-stage dynode plate is larger than that between said anode plate (5) and said last-stage dynode plate (6). An electron multiplier according to claim 6, further comprising an inverting dynode plate (13) for inverting orbits of secondary electrons passing through said electron passage holes of said anode plate (5), the inverting dynode plate (13) being spaced apart from said anode plate (5) by way of an insulating member and positioned such that said anode plate (5) is held between said inverting dynode plate (13) and said last-stage dynode of said dynode unit (60), said inverting dynode plate (13) having a depression on a main surface thereof opposing said anode plate (5) for receiving an insulating member in contact therewith at a contact portion, the anode plate (5) having a depression on a main surface thereof opposing said inverting dynode plate for receiving said insulating member in contact therewith at a contact portion, and wherein the distance between the contact portion of said inverting dynode plate (13) and the contact portion of said anode plate (5) is larger than that between said focusing electrode plate (7) and said anode plate (5). An electron multiplier according to claim 10, further comprising a shielding electrode plate (14) spaced apart from said inverting dynode plate (13) by way of insulating members and positioned such that said inverting dynode plate (13) is held between said anode plate (5) and said shielding electrode plate (14), said shielding electrode plate (14) having a depression provided on a main surface thereof opposing said inverting electrode plate (14) for receiving an insulating member in contact therewith at a contact portion, the inverting dynode plate (13) having a depression on a main surface thereof opposing said shielding electrode plate (14) for receiving said insulating member in contact therewith at a contact portion, and wherein the distance between the contact portion of said shielding electrode plate (14) and the contact portion of said inverting dynode plate (13) is larger than that between said shielding electrode plate (14) and said inverting dynode plate (13). An electron multiplier according to any preceding claim, wherein said insulating members (8a, 80a, 80b) are spherical bodies. An electron multiplier according to any one of claims 1 to 9, wherein said insulating members (8b) are circularly cylindrical bodies. A photomultiplier having an electron multiplier as set forth in any preceding claim, the photomultiplier further comprising a photocathode (3) for receiving photons and emitting photoelectrons, said dynode unit (60) being positioned between said photocathode (3) and said anode plate (5) for receiving photoelectrons emitted by said photocathode. A photomultiplier according to claim 11, further comprising: a housing (1) having a light receiving plate (2), said photocathode (3) being deposited on an inner surface thereof, the housing accommodating said dynode unit (60) and said anode plate (5); and a base member (4), to which said housing (1) is secured to form a vacuum container and having said dynode unit (60) mounted thereon, the base member supporting a plurality of connecting pins (11) to enable predetermined voltages to be applied to dynode plates (6) of said dynode unit (60). A photomultiplier according to claim 12, wherein a conductive metal for applying a predetermined voltage to said photocathode (3) is deposited on an inner wall of said housing (1), and said housing (1) and said photocathode (3) are rendered conductive by a predetermined conductive metal (12).

Inventors: ATSUMI AKIRA, HASEGAWA YUTAKA, KAWANO EIICHIRO, KUROYANAGI TOMIHIKO, KYUSHIMA HIROYUKI, MIZUIDE MASUYA, NAGURA KOJI, ATSUMI, AKIRA, HASEGAWA, YUTAKA, KAWANO, EIICHIRO, KUROYANAGI, TOMIHIKO, KYUSHIMA, HIROYUKI, MIZUIDE, MASUYA, NAGURA, KOJI
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1994
Main classification: H01J43
IPC codes: H01J43, H01J9